In a typical cellular radio system, wireless user equipment units (UEs) communicate via a radio access network (RAN) to one or more core networks. The user equipment units (UEs) may be mobile telephones laptop computers with mobile termination, and thus may be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network. Alternatively, the wireless user equipment units may be fixed wireless devices, e.g., fixed cellular devices/terminals which are part of a wireless local loop or the like. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a radio base station (RBS) such as a base transceiver station (BTS) or a NodeB/eNodeB. Each RBS communicates over an air interface with UEs that are within range. In the RAN, for 2G and 3G, multiple RBSs are typically coupled such as by landline or microwave to a control node known as a base station controller (BSC) or a radio network controller (RNC). The control node supervises and coordinates various activities of the multiple RBSs coupled thereto. Control nodes are typically coupled to one or more core networks. One example of a RAN is the Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (UTRAN). UMTS is a third generation system which builds upon the radio access technology known as Global System for Mobile communications (GSM). UTRAN provides wideband code division multiple access (WCDMA) to UEs. Further evolution of mobile technology has moved towards Long-Term Evolution (LTE) and LTE Advanced technologies having similar architectures.
In many RANs, each RBS is located at a single site. However, RBSs may have a distributed architecture. For example, a distributed RBS architecture may take the form of one or more radio equipment (RE) portions that are linked to a radio equipment control (REC) portion over an internal RBS interface.
Such distributed RBS architecture may have a main processor at each REC, and a set of antennas with dedicated RF equipment able to cover multiple radio cells at each RE, where the main processor may be shared among multiple REs. This architectural may provide higher capacity, lower cost, and lower latency between the RECs and the REs. The Common Public Radio Interface (CPRI) is an example of an internal RBS interface that links an RE portion of the radio base station to a radio equipment control portion of the base station. Other interfaces may be used, for example the Open Base Station Architecture Initiative (OBSAI).
This approach of providing “remotization” of the RE portion of the RBS from the REC portion may bring some notable advantages, including:
a) rationalization of RBS processing unit, with benefits in terms of cost and power consumption;
b) dynamic allocation of RF and/or processing resources depending on cell load and traffic profiles; and
c) correlation of data supported by all the antennas which are afferent on the same processing unit, which increases radio link reliability, bandwidth, and coverage and optimizes the power consumption.
This may enable some “cloud computing” concepts to be applied to the RANs. Point to point (P2P) optical links are currently generally used for the interface between the REC and the RE. For this interface, WDM systems, especially the ones used in the access (WDM-PON), may be used as they may enable guaranteed low latency, protocol transparency, high bandwidth and an increased spectral efficiency. Notably CPRI has pressing constraints in terms of latency (round-trip delay) and in particular in terms of uplink/downlink synchronization.
The CPRI standard recites optical fibers for transmission link up to ten kilometers (10 km), recites determining a round trip delay, and specifies synchronization and timing accuracies, e.g. link round trip delay accuracy of sixteen nanoseconds (16 nsec.).
As previously mentioned, an RBS is functionally separated into an RE and an REC. An RE may also be referred to as a radio resource unit (RRU). An REC may also be referred to as a digital unit (DU) or a baseband unit (BBU). The RE converts a transmit baseband signal to a transmit radio frequency (RF) signal and then feeds the transmit RF signal to an antenna. Further, the RE receives an RF signal from the antenna and converts the received RF signal to a received baseband signal. The REC generates the transmit baseband signal and processes the received baseband signal.
In a conventional wireless mobile network, the antenna, RE and REC are typically integrated into a single network node and placed at a cell site. As data rates and the number of subscribers continue to increase, the coverage area of typical cell sites may decrease. As the coverage area decreases, additional cell sites may be required. However, adding cell sites results in higher costs for network operators, since each cell site typically includes a source of power, real estate and a cell tower.
To address these issues, a RAN architecture has been developed to allow each RE to be geographical separated from its corresponding REC. By doing so, each REC may be co-located at a centralized location while each RE may be located at a corresponding cell site. An RE located at a cell site may also be referred to as a remote radio unit (RRU). Further, a point-to-point link may be used to couple a RE and an REC.
An interface specification for communications between an RE and an REC has been developed. The most widely adopted specification is the CPRI such as described by CPRI Specification Version 6.1. The CPRI specification describes the interface between a RE and an REC of the RBS system. FIG. 1 illustrates a CPRI reference scheme in an RBS system.
Current industry trends show a large number of REs distributed over a geographical area, with their corresponding RECs pooled at a centralized location, which is also referred as a baseband hotel. Further, the centralized pool of RECs is used to control the large number of REs distributed over the geographical area. This network architecture is commonly referred as C-RAN, which may also be referred to as cloud RAN, centralized RAN or coordinated RAN. The term cloud RAN is typically used to emphasize that RAN functions are virtualized in a centralized off-the-shelf server. The term centralized RAN is typically used to emphasize that RAN is managed by pooling RECs in a centralized location, which may also be referred to as an REC pool site. The term coordinated RAN is typically used to recognize the benefit of having RECs in the same location for coordination and cooperative transmission/reception of data.
In a C-RAN system, mobile traffic such as CPRI traffic is communicated over a communication link between the REC pool site and each RE site. This communication link is typically over a long distance and may also be referred to as a fronthaul. The communication link may have a forward link from the REC to the RE and a reverse link from the RE to the REC. For CPRI traffic, the communication link must meet certain performance requirements such as latency, jitter and symmetry. In particular, the overall latency over the network is typically within about one hundred microseconds (100 usec.). Further, the asymmetry between the forward and reverse transmissions is typically on the order of tens of nanoseconds. Also, the jitter is typically a few parts per billion.
The requirement for a symmetric network is associated with the need to calibrate for the overall delay. Further, the ability to meet this requirement becomes more challenging for a communication link using packet communications. This requires the reverse traffic packet and the corresponding forward traffic packet between the REC and the RE to be symmetric within a requirement of a few nanoseconds. For example, a thirty-two nanosecond (32 nsec.) asymmetry corresponds to sixteen nanoseconds (16 nsec.) error in the measurement of the overall delay. In fact, link delay calibration may be performed using a calculation of the round trip delay. FIG. 2 illustrates calibration of link delay in an RBS system using CPRI.
With these requirements, the transport of CPRI traffic over a traditional packet network in an RBS system is impractical due to at least packet delay variation (PDV) and asymmetry. However, the use of packet technologies to transport fronthaul traffic provides the ability to optimize resources among other things.
Various standards setting projects are ongoing to define the transport of time-sensitive traffic such as by using scheduled traffic principles described in IEEE802.1bv. In one example, IEEE802.1bv describes techniques to minimize or remove the impact of buffers on time sensitive traffic. Further, FIG. 3 illustrates establishing a guard band as described by IEEE802.1bv.
In another example, IEEE802.1 (P802.1Qbu) is defining mechanisms to support frame pre-emption in IEEE802.1Q bridges such as interrupting and delaying the transmission of a frame to allow transmission of a higher priority frame. This bridging mechanism relies on media access control (MAC) interfaces defined in IEEE802.3br.
Other similar initiatives have also commenced within the Deterministic Network Working Group (WG) of the Internet Engineering Task Force (IETF).
In addition, standards projects are ongoing to redefine the CPRI specification, including removing overhead from the CPRI traffic by reassigning functions from the REC to the RE. These projects assume that timing information is carried out-of-band such as by using IEEE1588 and not by the CPRI traffic per current practices. This assumption allows for various implementation options. For instance, CPRI traffic may be retimed at the edge of a packet network to filter out PDV and to equalize delays such as processing delays on both forward and reverse directions. This method may be implemented in the RE or external to the RE such as to support a legacy RE.
Furthermore, the transport of CPRI traffic over a packet network requires specific mapping procedures. For instance, the IEEE1904.3 standards group is investigating mechanisms analogous to the PWE (Pseudo-Wire Emulation) CW (Control Word). Further, generalized associated channel (G-ACh)-based residence time measurement (RTM) may be used by time synchronization protocols to transport packets over a multi-protocol label switching (MPLS) domain, in practice applying similar concepts to the transparent clock functions defined by IEEE1588. Specific aspects of this technique associated with 2-step clock principles have been patented by Ericsson, the assignee of the present application.
Existing solutions for the transport of CPRI traffic over a traditional packet network in a C-RAN system suffer from a number of problems. For instance, the transport of CPRI traffic over a packet network is currently not feasible due to PDV and asymmetry induced by the packet technologies. These problems lead to exceeding the stringent timing requirements of the CPRI traffic. A solution to these problems is to allow the transport of timing sensitive information, which is currently being discussed in the IEEE802.1 Time-Sensitive Networking (TSN) group. Some of the proposals for this solution require time synchronization by the packet nodes, apply scheduled traffic principles, and assume that all packet nodes are synchronized by IEEE1588-PTP. In practice, a time division multiplexing (TDM) approach is assumed such as having fixed timeslots allocated to specific traffic classes over a packet network. Such a solution may result in meeting a PDV requirement and having controlled latency.
However, the use of these principles is not sufficient to carry CPRI traffic over a packet switched network (PSN). In particular, the IEEE802.1 specifications may not fully address latency or PDV (packet delay variation). For instance, contention among traffic belonging to the same traffic class such as two CPRI traffic flows being contemporaneously received by the same node is not fully addressed even though a common case in CPRI scenarios. Further, contention among CPRI and non-CPRI traffic flows is also not addressed, preventing the use of multi-purpose packet nodes.
As previously mentioned, asymmetry may be corrected if the precision time protocol (PTP) is distributed to all nodes, as assumed in the IEEE802.1bv specification. By doing so, the actual end-to-end delays, in both directions, may be measured and used to compensate for asymmetry. However, requirements for delivering PTP to all network nodes might be an issue considering the level of accuracy required. Accordingly, there is a need for improved techniques for transporting internal RBS interface information over a packet switched network. In addition, other desirable features and characteristics of the present disclosure will become apparent from the subsequent detailed description and claims, taken in conjunction with the accompanying figures and the foregoing technical field and background.
The Background section of this document is provided to place embodiments of the present disclosure in technological and operational context and to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.